Blood is a liquid tissue composed of plasma, red blood cells, white blood cells, and platelets. It delivers nutrients and oxygen to cells while removing waste, helps regulate temperature and pH, and facilitates clotting. Red blood cells contain hemoglobin and carry oxygen, white blood cells protect against infection, and platelets help the blood clot. To donate blood, a person must be in good health, between 16-65 years old, weigh at least 110 pounds, and meet other criteria such as adequate hydration and rest.